The ability to non-destructively measure the thickness and dopant profiles in semiconductors has long been sought by the integrated circuit (IC) industry. In fact, dopant profile measurement techniques have been a subject of investigation for over 30 years. In one technique, referred to as Secondary Ion Mass Spectroscopy (SIMS), the dopant profile of a semiconductor is measured as it is incrementally destroyed layer by layer. As a result, a dopant profile is obtained, however, the semiconductor material is lost.
It is common knowledge in solid state physics that the introduction of dopants into a semiconductor alters its optical properties in the infrared spectral range due to the presence of free carriers. At the simplest conceptual level, free carriers contribute to the optical constants n and k as described by the well known Drude model. Thus far, optical measurement techniques can be classified into two categories, namely, the Infrared Reflectance (IR) technique and the Fourier Transform Infrared (FT-IR) Interferometry technique.
Infrared Reflectance (IR) Technique
The IR technique was first used in 1960 to measure the thickness of silicon epitaxial (epi) layers. The technique exploits the existence of optical contrast in the infrared spectrum due to different doping levels in a lightly doped epi-layer and a heavily doped substrate. The different doping levels cause interference when IR light is reflected from the surface of the sample. For an epi-layer exceeding 2 micrometers (.mu.m) in thickness, the reflectance waveform produces oscillatory behavior allowing the film thickness to be derived from the distance between the adjacent interference fringes. The technique has a number of disadvantages, the main one of which is that the position of the interference fringes is strongly influenced by the substrate dopant concentration, as well as the disappearance of the fringes altogether for sub-1 .mu.m epi-layers. There have been attempts to improve the technique by accounting for the phase changes upon the reflection at the epi/substrate interface. One theory calculated such changes using classical Boltzmann statistics, however the computations failed to agree with experimental results across the broad IR frequency range of 5-40 .mu.m. The computations also failed to agree with experimental results wherein the phase shift correction is particularly significant for thin epi-layers. Attempts have also been made to extend the IR reflectance technique to thin (0.5 .mu.m) epi-layers by comparing the Drude model with other known models. It was found that the Drude model is more applicable to epi-layers on heavily doped substrates, such as 2E19 cm.sup.-3, while other models are more accurate for lightly doped substrates, such as 5E18 cm.sup.-3. No model was able to adequately describe both cases. Currently, the IR technique is only applicable to the measurements of epi-layers thicker than 2 .mu.m with substrate resistivity less than 0.02 .OMEGA.-cm and epi-layer resistivity less than 0.1 .OMEGA.-cm.
Fourier Transform Infrared (FT-IR) Interferometry Technique
The FT-IR technique has found wide-spread use as a powerful tool for chemical analysis of materials where various material properties can be inferred from their infrared absorbance spectra. The application of FT-IR for film thickness determination was introduced in 1972 for measurements of thin polymer films and has since been widely adopted by the IC industry as the standard method for epi-layer thickness measurements. Unlike the IR technique, which uses dispersive infrared spectrophotometry, this method uses FT-IR in an interferogram mode. An instrument implementing an FT-IR consists of a Michelson interferometer coupled to a computer system. A Michelson interferometer divides a beam of radiation from an incoherent infrared source into two paths and recombines them at a detector after a path difference has been introduced, creating a condition under which an interference between the two beams can occur. The intensity variation as a function of the path difference is captured by the detector and results in the interferogram.
A typical interferogram consists of a strong center burst and two similar smaller bursts positioned symmetrically to the sides of the center burst. The epi-layer thickness is determined according to the formula: ##EQU1##
where d is the epi-layer thickness, 2.DELTA. is the distance between the side-bursts in the interferograms (same as the path difference between the two beams), n is the refractive index of the epi-layer, and .theta. is the angle of refraction in the epi-layer. However, as the film thickness decreases, the side-bursts move into the strong center burst until they get completely obscured, making the epi-layer measurement by side-burst identification impossible. This occurs when the epi-layer thickness is reduced below approximately 1 .mu.m. Attempts at extending the interferogram measurements to thinner films by utilizing a center-burst cancellation technique, wherein an interferogram of a matched substrate is subtracted from the initial measurement, have produced very limited success. Even if a perfectly matched substrate could be found, this still does not account for the secondary contribution to the center-burst formation due to the epi-layer presence, nor are the frequency responses of the instrument's optical and electronic components and the material properties taken into consideration. These items create phase shifts in the interferogram which influence the shape and absolute and relative positions of the side-bursts. Even in the cases where the film thickness is sufficient for side-burst identification, these phase shifts cause enough of an error to make film thickness measurements approaching 1 .mu.m increasingly uncertain.
In view of the above, what is needed is an improved technique to obtain an accurate non-destructive measurement of film thickness or dopant concentrations of doped semiconductors. This includes such semiconductor structures as silicon epitaxial layers on silicon substrates where the epi-layer has a different doping level from the substrate, for example, an undoped or lightly doped epi-layer on a heavily doped substrate. The technique should also work for structures having an ion-implanted or diffused profile, where a layer of dopants is introduced into a semiconductor having a lighter dopant level, including such specific structures as buried layers and shallow junctions. In both of these examples, the improved technique should enable one to determine the thickness of the epitaxial or implanted layer, the thickness of the transition layer between the film and substrate, and the concentration of free carriers in the film and the substrate, without having to destroy the sample in the process. In addition, the technique should account for imperfections in the measuring device and yield accurate results for sub-1 micron epi-layers.